Back-up control apparatus for turbo machine

ABSTRACT

A method and apparatus for controlling a turbo-machine in case of failure of the main burn fuel flow calculator. The apparatus receives at least information from various sensors as well as from the main burn fuel flow calculator. The method and apparatus, herein disclosed, provides a smooth and secure transition between the last burn fuel flow value before the failure of the main burn fuel flow calculator and the future burn fuel flow values.

FIELD OF INVENTION

This invention relates to a turbo-machine control system and moreprecisely to a digital electronic back-up control system.

BACKGROUND OF INVENTION

Modern aircraft engines are driven by an electronic engine controlsystem, which controls the thrust and the torque delivered by theengine. Usually such systems are designed using a microprocessor,allowing the control algorithms to be implemented by software.

The electronic engine control system must at least detect an overspeedof the turbo-machinery that could be fatal for it. In such a case, it isimportant to cut off the fuel flow.

These control algorithms control the spool speed of the turbo-machineryby scheduling the bum fuel flow through a hydromechanical fuel meteringunit. Sensors on the turbo-machine supply information on what is neededto drive the turbo-machine. The control algorithms which are running onthe microprocessor provide the proper bum fuel flow using all theinputs.

In case of emergency, it is important to rely on a back-up system. Suchback-up systems will provide the same information to the fuel meteringunit, Two different configurations are now available.

In the first configuration available, the back-up system is ahydromechanical back-up system. In this configuration, the back-upsystem controls the bum fuel flow through a mechanical, hydraulic orpneumatic control system.

In the other configuration available, the back-up system is anotherelectronic engine control system identical to the main electroniccontrol system. Such a parallel system is named a dual channelelectronic engine control system. A special algorithm might be used toensure a proper switch between one channel and the other.

Furthermore, the use of a hydromechanical back-up control increases theweight and the complexity of the control system, thus this is not anadvantageous solution for a use in an aircraft.

When a failure of the electronic engine control system is detected, asudden transition from the electronic engine control system to a back-upengine control system can be harmful to the turbo-machine. In fact, ifthe fuel flow computed by the electronic engine control system at itsfailure is far different from the fuel flow computed by the back-upengine control system when it is enabled, great damages can occur due toa the non-continuous supply of fuel.

Therefore there is a need for a reliable back-up control system thatwill smoothly take control of the turbo-machinery in the event of afailure.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electronicallyoperated back-up system for a turbo-machine to be use in case of failureof the main electronic engine control system.

It is another object of the present invention to provide anelectronically operated back-up system for a turbo-machine that is not aduplication of the main electronic engine control system.

It is an object of the present invention to provide an electronicallyoperated back-up system for a turbo-machine that is able to start theengine.

It is another object of the present invention to provide anelectronically operated back-up system for a turbo-machine that is ableto guarantee a minimum fuel flow in the engine in order to avoid aflameout.

It is another object of the present invention to provide anelectronically operated back-up system for a turbo-machine that is ableto avoid power increase or decrease in the engine during the transfer tothe back-up control mode.

According to one aspect of the invention, there is provided a back-upsystem for controlling a turbo-machine via a fuel metering unit when theelectronic engine control of the turbo-machine fails, which comprises aback-up burn fuel flow calculator, the back-up fuel flow calculatorreceiving at least the amount of fuel requested by the operator signalof the turbo-machine and the actual spool speed of the turbo-machine andcomputing an amount of fuel to provide to said turbo-machine in order toadjust its speed to obtain a spool speed equivalent to the amount offuel requested by the operator signal, a last burn fuel flow storage,the last burn fuel flow storage storing the last burn fuel flow valueprovided by the electronic engine control before its failure, atransitional burn fuel flow manager, the transitional burn fuel flowmanager receiving the amount of fuel computed by the back-up burn fuelflow calculator and the last bum fuel flow value stored in the storageand providing an amount of fuel to provide to the fuel metering unit.

According to another aspect of the invention, there is provided a methodfor determining a burn fuel flow value, comprising the steps ofreceiving a current fuel flow value provided to a fuel metering unit ofa turbo-machine from a main burn fuel flow calculator, storing thecurrent fuel flow value in a memory upon detection of a malfunction ofthe main burn fuel flow calculator, calculating a transitional valueusing the stored fuel flow value and a back-up burn fuel flow valuereceived from a back-up burn fuel flow calculator and outputting thetransitional value to said fuel metering unit.

According to another embodiment of the present invention, there isprovided a method for determining a burn fuel flow value for an aircraftturbo-machine in case of a malfunction, the method comprising the stepsof receiving a current fuel flow value from a main burn fuel flowcalculator, storing the current fuel flow value prior to themalfunction, calculating a back-up burn fuel flow value using sensorreadings from the aircraft and initially the stored current fuel flowvalue, wherein the calculating using an algorithm which is differentfrom an algorithm used for calculating the main bum fuel value, theback-up burn fuel flow value having a smooth transition between thecurrent fuel flow value prior to the malfunction and future burn fuelflow values and outputting the back-up burn fuel flow value to a fuelmetering unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by an examination of thefollowing description, together with the accompanying drawings, inwhich;

FIG. 1a shows a block diagram of one embodiment of the present inventionwherein a turbo-machine is driven by either an electronic engine controlor a back-up control via a fuel metering unit;

FIG. 1b shows a block diagram of another embodiment of the presentinvention wherein a turbo-machine is driven by either an electronicengine control or a back-up control via a fuel metering unit; anoverspeed control ensures that take proper measures are taken in case ofan overspeed detection;

FIG. 2 shows a flow chart of the operations performed by an electronicengine backup control; in which a test is first performed to know if theback-up control is selected and in such a case, a test is performed tofind out if this is the first time the back-up control has been selectedwhile various operations are performed in each case in order to drivesafely the turbo-machine;

FIG. 3 shows a flow chart of the operations performed to compute theamount of fuel when the transition between the electronic engine controland the electronic engine back-up control is finished;

FIG. 4 shows a flow chart of the operations performed to compute thefuel needed;

FIG. 5 shows a flow chart of the operations performed to compute thefuel needed in one case in which an increment is added to a computedvalue; and

FIG. 6 shows a flow chart of the operations performed to compute thefuel needed in one case in which an increment is subtracted to acomputed value.

PREFERRED EMBODIMENT

Now referring to FIG. 1a, there is shown a turbo-machine 30. Thisturbo-machine 30 receives fuel from a fuel metering unit 20. The fuelmetering unit 20 is fed by fuel which comes from the fuel tank of theaircraft. The fuel metering unit 20 is either driven by a main burn fuelflow calculator 14 or by a back-up control 25. In case of failure of themain burn fuel flow calculator 14, the back-up control 25 provides theburn fuel flow request to the fuel metering unit 20. Both the main burnfuel flow calculator 14 and the back-up control 25 collect informationcoming from a group of sensors 35. The type of information used by themain burn fuel flow calculator 14 and the back-up control 25 will bemore detailed in the following description.

More precisely, and still referring to FIG. 1a, the group of sensors 35comprises a Throttle Lever Angle (TLA) sensor 10, an ambient pressure(P_(Amb)) sensor 12, an ambient temperature (T_(Amb)) sensor 11 and aturbo-machine speed (N) sensor 13. A watchdog 16 receives the signalsfrom the group of sensors 35. The watchdog 16 also receives the burnfuel flow request from the main bum fuel flow calculator 14. Thewatchdog 16 is then able to detect any failure from the main burn fuelflow calculator 14 knowing its inputs and its outputs. In case of adetection of a failure, the main burn fuel flow calculator 14 willinform the back-up control 25 and a multiplexer 19. The multiplexer 19is responsible for selecting between the burn fuel flow request comingfrom the back-up system 15 and the burn fuel flow request coming fromthe main burn fuel flow calculator 14.

The back-up system 25 comprises a back-up bum fuel flow calculator 15, astorage of the last burn fuel flow 17 and a transitional burn fuel flowmanager 18. The burn fuel flow calculator 15 may, in one embodiment ofthe present invention, be identical to the main burn fuel flowcalculator 14. The purpose of the back-up burn fuel flow calculator 15is to provide the amount of fuel that will allow the turbo-machine toadjust his speed in accordance with the TLA 10 chosen by the pilot ofthe aircraft. In case of a failure of the main burn fuel flow calculator14, detected by the watchdog 16, the last burn fuel flow is stored bythe last burn fuel flow storage 17. A transitional burn fuel flowmanager 18 is enabled in such a case. This transitional burn fuel flowmanager 18 will provide a burn fuel flow request value that will beequal to a value which is a transition from the last burn fuel flowvalue which is stored by the last burn fuel flow storage 17 and a valuecomputed by the back-up burn fuel flow calculator 15. This will ensure asmooth transition from the time of failure to the time of recovery bythe back-up burn fuel flow calculator 15.

Now referring to FIG. 1b, there is shown a back-up control 25. Theback-up control 25 comprises an overspeed control 21. The overspeedcontrol 21 receives the spool speed from a group of sensors 35. In caseof an overspeed detection, the overspeed control 21 which controls thefuel metering unit 20 imposes a shutdown on the fuel delivery in theturbo-machine 30 via the fuel metering unit 20. In the preferredembodiment of the present invention, the group of sensors 35 comprisesvarious identical sensors in order to provide safe information in caseof failure of one sensor. Still referring to FIG. 1b, and for a sake ofclarity, there is only shown two identical sensors for the spool speedmeasurement which are the spool speed sensor 13 and the overspeed spoolspeed sensor 22. In this embodiment, the back-up bum fuel flowcalculator 15 receives also a delayed fuel flow value from a delay unit26 and a signal from the watchdog unit 16. The back-up burn fuel flowcalculator 15 also receives its computed burn fuel flow request valueusing feedback signal 24. These two features enable for instance astart-up of the engine in case of a failure of the main burn fuel flowcalculator 14. In this embodiment of the invention a broad variety ofcontrol strategies may be implemented. The delay unit 26 helps toprovide the last fuel flow value validated in order to offer anon-corrupted value in case of failure of the sensors; such a delay maybe around 40 msec.

Now referring to FIG. 2, there is shown a flowchart which indicates theoperations performed by the transitional burn fuel flow manager 18.

According to step 67 of the present invention, a test is performed bythe transitional burn fuel flow manager 18 in order to check if theback-up mode was selected. The back-up mode is defined as the periodbetween the failure of the main bum fuel flow calculator 14 and the endof the tracking. If a back-up mode was selected, a test is performedaccording to step 65 of FIG. 2 to know if this is the first time theback-up mode is selected (i.e. the failure of the main burn fuel flowcalculator 14 has just happened). If this is the first time that theback-up mode is selected and according to step 66, the fuel flow requestis equal to the latest burn fuel flow request. The transitional burnfuel flow manager 18 uses the information stored inside the last burnfuel flow manager to obtain the latest burn fuel flow request. Accordingto step 68, various calculations are then performed. Now referring toFIG. 4, there is shown the calculation performed during step 68.According to step 74, the fuel needed (F_(N)) is computed. Thecomputation of the fuel needed (F_(N)) during step 74 is detailed inFIG. 3. This computation is performed by the back-up burn fuel flowcalculator 15. According to FIG. 3, the TLA signal is read by theback-up burn fuel flow calculator 15. An access to a look-up table isdone according to step 86. According to step 87, the TLA signal providesa speed reference (N_(Ref)) using the access to the look-up table. Theactual speed of the turbo-machine is read according to step 88 and usingthe signal coming from the spool speed sensor 13. At this point, thedifference between the speed reference and the actual speed is computedaccording to step 89. An access to a look-up table is performedaccording to step 90. Finally, the fuel needed is determined using thedifference computed and the access to the look-up table.

Now referring back to FIG. 4, once the fuel needed is determined, asexplained above, the difference between the latest bum fuel flow request(F_(Validated)) from the main burn fuel flow calculator 14 and the fuelneeded (F_(N)) is computed according to step 101.

Now referring back to FIG. 2, a test is performed at step 69 on theresult of the difference computed (F_(Validated)−F_(N)) 79. If theresult is equal to zero, the tracking that ensures a smooth changebetween the latest burn fuel flow request from the storage 17 and theburn fuel flow request value given by the back-up proportional controlloop from the back-up burn fuel flow calculator 15 is finished.According to step 70, the variable that indicates that a back-up mode isselected is disabled. According to step 75, the fuel needed (F_(N)) canthen be provided to the fuel metering unit 20 via the multiplexer 19.

If the result is not equal to zero, a test is performed according tostep 71. This test consists in finding out if the result of thedifference computed (F_(Validated)−F_(N)) 79 is superior to 0. If thisis the case, i.e. if the latest fuel flow request is superior than thefuel needed, the amount of fuel to be provided will be computedaccording to step 73. Now referring to FIG. 6, there is shown thedifferent steps performed during step 73. According to step 110, thespool speed of the turbo-machine (F_(N)) is loaded into a singlevariable (F_(Act)). According to step 111, an increment is subtractedfrom the value (F_(Act)) and the new value is stored under the singlevariable (F_(Act)).

Now referring back to FIG. 2, if the difference computed(F_(Validated)−F_(N)) 79 is inferior to 0, the amount of fuel needed iscomputed according to step 72 of FIG. 2. Now referring to FIG. 5, thereis shown, more precisely, the different steps performed during step 72.According to step 105, the spool speed of the turbo-machine (F_(N)) isloaded into a single variable (F_(Act)). Then, according to step 106, anincrement is added to this single variable (F_(Act)) and the new valueis stored under the single variable (F_(Act)).

In the case that the difference computed (F_(Validated)−F_(N)) 79 isinferior to 0, the single variable computed (F_(Act)) is used to providethe amount of fuel according to step 75. Similarly, in the case that thedifference computed (F_(Validated)−F_(N)) 79 is superior to 0, thesingle variable computed (F_(Act)) is used to provide the amount of fuelaccording to step 75.

In this embodiment of the present invention, there is disclosed a way tocontrol efficiently a turbo-machine in case of a failure of a main burnfuel flow calculator. This control is performed bearing in mind that thetransition between the main burn fuel flow calculator 14 and the back-upburn fuel flow calculator 15 must be smooth.

What is claimed is:
 1. A back-up system for controlling a turbo-machinevia a fuel metering unit when a main electronic engine control of theturbo-machine fails and which comprises; a back-up burn fuel flowcalculator, said back-up fuel flow calculator receiving at least theamount of fuel requested by an operator signal of the turbo-machine andthe actual spool speed of the turbo-machine and computing an amount offuel to provide to said turbo-machine in order to adjust its speed toobtain a spool speed equivalent to said amount of fuel requested by theoperator signal; a last burn fuel flow storage, said last burn fuel flowstorage storing the last burn fuel flow value provided by said mainelectronic engine control before its failure; and a transitional burnfuel flow manager, said transitional burn fuel flow manager receivingthe amount of fuel computed by said back-up burn fuel flow calculatorand said last burn fuel flow value stored in said storage and providinga transitional amount of fuel to said fuel metering unit.
 2. Theapparatus as claimed in claim 1, further comprising a non-volatilememory connected to said back-up burn fuel flow calculator.
 3. Theapparatus as claimed in claim 2, wherein said back-up burn fuel flowcalculator receives also an ambient temperature and an ambient pressurefrom an ambient temperature sensor and an ambient pressure sensor. 4.The apparatus as claimed in claim 3, wherein said non-volatile memorycomprises a look-up table which provides a relation between a fuelconsumption of the turbo-machine and the speed of the turbo-machine,using said ambient temperature and said ambient pressure.
 5. Theapparatus as claimed in claim 1, wherein said transitional burn fuelflow manager receives an enabling signal from said main electronicengine control of the turbo-machine, said enabling signal beingresponsible for beginning said computing of the amount of fuel toprovide to said fuel metering unit.